1. Field of the Invention
The invention relates to a process for producing aluminosilicate glass, which, apart from inevitable impurities, is free of alkali metals and contains more than 12% by weight of Al2O3 (in % by weight, based on oxide content), with addition of a fining agent to the batch formulation.
2. Description of the Related Art
Processes for producing glass comprise the process steps of batch formulation, also known as batch charging, melting of the glass and subsequent hot-forming thereof. In this context, the term melting also encompasses the steps of fining, homogenizing and conditioning for further processing, which follow the melting-down operation.
With regard to melts, the term fining is understood as meaning the removal of gas bubbles from the melt. In order to achieve a very high level of freedom from extraneous gases and bubbles, it is necessary for the melted batch to be thoroughly mixed and degassed. The characteristics of gases and bubbles in the glass melt, and their removal, are described, for example, in “Glastechnische Fabrikationsfehler” [glass-making defects], edited by H. Jebsen-Marwedel and R. Brückner, 3rd Edition, 1980, Springer-Verlag, pages 195 ff.
There are two fundamentally different fining processes, which are common knowledge; they differ essentially in the way in which the fining gas is produced:
In the case of mechanical fining, gases such as water vapor, oxygen, nitrogen or air are injected through openings in the bottom of the melting unit. This process is known as bubbling.
The most frequent fining processes are the chemical processes. Their principle consists in adding to the melt or even to the batch                compounds which decompose in the melt and in doing so release gases, or        compounds which are volatile at relatively high temperatures, or        compounds which give off gases at relatively high temperatures in an equilibrium reaction.        
As a result, the volume of existing bubbles is increased and their distension is intensified. The last-named compounds include what are known as redox fining agents, such as antimony oxide and arsenic oxide. In the case of this process, which is the most frequently used in practice, the redox fining agents employed comprise polyvalent ions which are able to exist in at least two oxidation states and which are in a temperature-dependent equilibrium with one another; at high temperatures a gas, usually oxygen, is released.
The second group, made up of compounds, which are volatile at high temperatures owing to their vapor pressure and so exert their effect, includes, for example, chlorides, for example sodium chloride, and various fluorides. These are referred to collectively as evaporating fining agents.
The redox fining and the evaporating fining are linked to the temperatures at which, on account of the thermodynamic conditions, the corresponding redox or evaporating (or sublimation) processes take place. For many glass melts, such as the melts of soda-lime glasses and other relatively low-melting glasses (e.g. borate glasses, lead glasses), these options are sufficient.
However, in the case of glasses with melting points (temperature at which the viscosity is approx.102 dPas) of between approx. 1550° C. and 1650° C., which for sufficient fining means fining temperatures of more than 1600° C., the increased viscosity of the glass melt means that the formation of the bubbles deteriorates. The tendency of the bubbles to grow is reduced and they are less able to rise than at lower viscosities. Therefore, fine bubbles are formed, and these bubbles can no longer be removed or can only be removed with very great difficulty, even by reducing the throughput or by using higher temperatures, making glasses of this type unusable, since the resorption action of the chemical redox refining agent, e.g. of the Sb2O3, i.e. the ability to resorb the oxygen or other gases from the fine bubbles and thereby remove them during cooling, is insufficient for many high-melting glasses.
The possibilities for increasing the temperatures in order to reduce the viscosity and lengthen the melting and fining times, which in principle do exist to a certain extent, are also not economical, since, by way of example, the latter measure would make the melting capacities too low.
A further drawback of many redox fining agents and evaporating fining agents is that they are environmentally harmful or at least are not environmentally friendly. This is true, for example, of fluorides, of arsenic oxide and also of antimony oxide. Alternative redox fining agents, for example cerium oxide, are relatively expensive replacement substances.
The high-melting glasses mentioned above include primarily alkali-free aluminosilicate glasses containing more than 12% by weight of Al2O3, in particular those which contain little or no B2O3, in particular aluminosilicate glasses as used, because of their high thermal stability which is associated with the high melting points, as substrate glasses, for example in display technology, or in particular as lamp glasses, for example for halogen lamps.
In the case of glasses for halogen lamps, a further drawback of Sb2O3 consists in the fact that, at least in relatively high proportions, it promotes the undesirable blackening on the inner side of the bulb, which is caused by tungsten deposition resulting from disturbances to the regenerative halogen cycle, and that during hot flame processing the pinch, i.e. the fusion between glass and supply conductor wires, acquires a brown discoloration on account of the reduction of the antimony oxide.
The first type of chemical fining, i.e. fining by means of compounds, which decompose and in doing so release gases, includes sulfate fining. This fining too is known for low-melting glasses, such as soda-lime glasses, since the commonly used Na2SO4 (in the case of mass-produced glasses, also in the form of Glauber's salt Na2SO4 10H2O) reacts with the SiO2 which is always present at temperatures, which, in comparison with Na2SO4, which is relatively stable on its own, are low, in accordance with equation (1) or (2):Na2SO4+SiO2→Na2O.SiO2+SO2+½O2  (1),orNa2SO4+Na2S+SiO2→2 Na2O.SiO2+SO2+S  (2)
The action of the sulfate fining is highly dependent on the chemical solubility of SO3 or of SO2−4. Dissolved SO3 in the glass does not have a fining action, but rather leads to seeding (seed formation) in the glass, i.e. to fine bubbles, or, in the event of super saturation, to the formation of salt bubbles. These are to be understood as meaning pools, predominantly of molten sodium sulfate, which float on the glass flux. The fining action of the sulfate is only produced by reaction with reducing agents, which are also added, for example coal, blast-furnace slag or sulfides. In the process, SO3 reacts to form SO2 and ½O2. Unlike SO3, SO2 has a low solubility in the glass. It is precipitated and forms bubbles or promotes the growth of existing bubbles.
For example, both Jebsen-Marwedel and Bruckner (loc. cit., pp. 231 ff.) and J. Lange in “Rohstoffe der Glasindustrie” [raw materials in the glass industry], 3rd Edition, 1993, Verlag für Grundstoffindustrie, Leipzig, Stuttgart pp. 176 ff, describe the formation of seeds in oxidic glass melts. The formation of SO2 bubbles and their growth does not occur. The glass is not degassed or is only badly degassed, and a large number of fine bubbles or seeds remain. Sulfate fining is not suitable for glass melts of this type.
On the other hand, in view of the intended applications for the alkali-free aluminosilicate glass, reducing melting conditions or the addition of reducing agents are not possible. For example, in the case of halogen lamp glasses, the likelihood of blackening as a result of tungsten deposition would rise to such an extent that the glasses would become unusable for this application.
The patent literature has already disclosed aluminum-containing glasses from a broad range of compositions, which, in addition to other fining agents, may also contain sulfate.
For example, JP 10-25132 A describes glasses to which, in addition to SO3, chloride, given as up to 2% by weight of Cl2, is added, while JP 10-324526 A mentions glasses to which one component selected from the group consisting of Fe2O3, Sb2O3, SnO2, SO3 and one component selected from the group consisting of Cl and F is added and which still also contain As2O3.
Furthermore, alkali-containing glasses (U.S. Pat. No. 3,148,994; U.S. Pat. No. 5,631,195; EP 0 769 418 A1;JP 3-40933 A; WO 98/47111 A) and/or low-aluminum glasses (JP 55-3367 A; EP 0 769 481 A1), which are intended to be fined using sulfate, are known.
Moreover, EP 204 431 A2 has disclosed an arsenic-containing glass to which between 2.95 and 11.8% by weight of sulfate, expressed as SO3, is added. Such a high level of sulfate is used not as a fining agent but rather to form a structure in the network of the glass.